1. Field of the Invention
The invention relates to an analog-to-digital converter, and more particularly to a multiplying digital-to-analog converter with shared switches and a pipeline analog-to-digital converter using the same.
2. Description of the Related Art
In portable digital multimedia consumer electronic systems, analog-to-digital converters (referred to as ADCs) with high speed and low power consumption are required to process analog signals. Pipeline ADCs are ADCs with pipeline structures which can achieve high speed and high precision. The pipeline ADCs have sampling rates reaching dozens of trillions of sampled points per second; even reaching hundreds of trillions of sampled points per second. That is, the sampling rates are dozens of MS/s; even hundreds of MS/s. This characteristic is advantageous for pipeline ADCs, so pipeline ADCs are commonly used in consumer electronic systems.
FIG. 1A is a schematic view showing a conventional pipeline ADC. As shown in FIG. 1A, a pipeline ADC comprises a multi-stage pipeline circuit structure. The second pipeline circuit stage is given as an example for illustration (refer to the portion surrounded by a dotted line in FIG. 1A). The second stage pipeline circuit comprises a sample-and-hold (referred to as S/H) circuit, a sub ADC circuit, a sub digital-to-analog converter (referred to as sub DAC) circuit, a subtracter circuit, and a residue amplifier circuit. The sub ADC circuit is used to quantify an analog-signal input quantity Vin, perform an analog-to-digital conversion to the quantification result, and output a digital quantity (that is a binary digital signal) corresponding to the analog-signal input quantity Vin. The sub DAC circuit processes the digital quantity output from the sub ADC circuit and outputs an analog-signal quantity. The subtracter circuit performs a subtraction operation on the analog-signal input quantity Vin and the analog-signal quantity output from the sub DAC. Then, through an amplifying operation performed by the residue amplifier circuit, a residue signal Vout of the analog-signal input quantity Vin is obtained. The residue signal Vout serves as the analog-signal input quantity of the next pipeline circuit stage and is processed by the next pipeline circuit stage. In each pipeline circuit stage, the S/H circuit, the sub DAC circuit, the subtracter circuit, and the residue amplifier circuit are generally called as a multiplying digital-to-analog converter (referred to as MDAC).
FIG. 1B is a schematic view showing an MDAC in a conventional pipeline ADC. FIG. 1C shows a timing chart of switches in the MDAC of FIG. 1B. In a conventional pipeline ADC, for achieving sampling operations with various precision levels, sampling capacitors Cs with different numbers, feedback capacitors Cf with different numbers, sampling switches, decoding switches, and feedback switches have to be disposed in each MDAC. Specifically, as shown in FIG. 1B, an MDAC circuit comprises a sub DAC decoding circuit 10, a capacitor-switch circuit 20, and an operation amplifier circuit 30. The sub DAC decoding circuit 10 is coupled to an output terminal of the sub ADC circuit 40 of the current pipeline circuit stage and receives analog signals Vrp, Vcm, and Vrn, respectively through three input terminals. The switch-capacitor circuit 20 is coupled to an analog-signal input quantity Vin output from the previous pipeline circuit stage or S/H circuit, the sub DAC decoding circuit 10, and the operation amplifier circuit 30. The MDAC processes the analog-signal input quantity Vin and a digital quantity output from the sub ADC circuit 40 to obtain a residue signal of the analog-signal input quantity Vin. The residue signal is processed by the next pipeline circuit. The MDAC composed of the sub DAC decoding circuit 10, the capacitor-switch circuit 20, and the operation amplifier circuit 30 can carry out the functions of the S/H circuit, the sub DAC circuit, the subtracter circuit, and the residue amplifier shown in FIG. 1A. For achieving an MDAC circuit structure with 3.5 bits, the capacitor-switch circuit 20 requires seven sampling capacitors Cs, one feedback capacitor Cf, seven sampling switches φ1 respectively coupled to the sampling capacitors Cs, seven decoding switches φ2 respectively coupled to the sampling capacitors Cs, one switch φ1 coupled to the feedback capacitor Cf, and one feedback switch coupled to the feedback capacitor Cf, wherein the feedback switch is one decoding switch φ2. The sampling switches φ1 are coupled to the analog-signal input quantity Vin required to be processed. The seven decoding switches φ2 coupled to the sampling capacitor Cs are coupled to output terminals of the sub DAC decoding circuit 10. The feedback switch is coupled to the output terminal Vout of the operation amplifier circuit 30. The feedback capacitor Cf is coupled to the sampling switches Cs and an input terminal of the operation amplifier circuit 30. Further, the feedback capacitor Cf, the sampling switches Cs, and the input terminal of the operation amplifier circuit 30 are coupled to ground through a ground switch φ1e. The sampling switches φ1, the decoding switches φ2, and the ground switch φ1e can operate according to the timing of FIG. 1C to control their turned-on and turned-off states thereby achieving the functions of the S/H circuit, the sub DAC circuit, the subtracter circuit, and the residue amplifier shown in FIG. 1A. Thus, the analog-signal input quantity Vin of the current pipeline circuit stage can be processed to obtain the residue signal of the analog-signal input quantity Vin.
In an MDAC of a conventional pipeline ADC, each sampling capacitor Cs has to be coupled to one sampling switch and one decoding switch. The total number of switches is large, and the sampling switches and the decoding switch have greater sizes. Thus, the switches of the MDAC occupy a large area in the entire circuit.